Derivatives And Risk Management (Interest Rate Swap Agreements) (Details) $ in Thousands	3 Months Ended	9 Months Ended	12 Months Ended
	Sep. 30, 2017 USD ($) Caontracts	Sep. 30, 2017 USD ($) Caontracts	Dec. 31, 2016 USD ($) Caontracts
Derivatives, Fair Value [Line Items]
Payments for (proceeds from) derivative instrument operating activities	$ (8,800)
2017 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts			6
Derivative, Notional Amount			$ 75,000
Number of settled contracts | Caontracts	5
Settled Derivative, Notional Amount	$ 60,000
Derivative, Maturity Date			Dec. 31, 2017
2018 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	14	14	14
Derivative, Notional Amount	$ 275,000	$ 275,000	$ 275,000
Derivative, Maturity Date		Dec. 31, 2018	Dec. 31, 2018
2019 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	6	6	6
Derivative, Notional Amount	$ 70,000	$ 70,000	$ 70,000
Derivative, Maturity Date		Dec. 31, 2019	Dec. 31, 2019
2020 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	3	3	2
Derivative, Notional Amount	$ 30,000	$ 30,000	$ 20,000
Derivative, Maturity Date		Dec. 31, 2020	Dec. 31, 2020
2021 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	1	1
Derivative, Notional Amount	$ 15,000	$ 15,000
Derivative, Maturity Date		Dec. 31, 2021
2022 | Interest Rate Swap [Member]
Derivatives, Fair Value [Line Items]
Number of Interest Rate Derivatives Held | Caontracts	5	5	5
Derivative, Notional Amount	$ 60,000	$ 60,000	$ 60,000
Derivative, Maturity Date		Dec. 31, 2022	Dec. 31, 2022